RNA gain-of-function was established as a genetic mechanism through studies of myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2), where expression of expanded CUG repeat (CUGexp) or expanded CCUG repeat (CCUGexp) RNA leads to myotonic myopathy and multisystemic symptoms. Although mechanisms of RNA toxicity are multifaceted and complex, evidence is converging on sequestration of MBNL protein in nuclear foci of CUGexp or CCUGexp RNA as a pivotal event. MBNL proteins are multifunctional RNA binding proteins that regulate alternative splicing, turnover, and other aspects of gene expression. As MBNL proteins become trapped in foci, residual levels of MBNL function decline, leading to major disruptions of RNA processing. However, while the evidence for this mechanism is strong, it's unclear that it provides a unitary explanation, particularly in DM2, where Repeat Associated Non-AUG (RAN) translation may occur. Therapeutic strategies to mitigate RNA toxicity have shown major benefit in animal models, and new targeted treatments are advancing to clinical trials. To help ensure proper design of trials, and promote trial designs that are highly informative, there is a need to define natural history, clinical endpoints, and biomarkers. In this respect, substantial progress has been made to establish natural history of DM1, but DM2 has not been investigated. Similarly, major progress has been made to develop biomarkers of RNA toxicity in DM1, but more work is needed to qualify these measures as drug development tools, and to assess their application to DM2. As a first step to establish natural history for DM2, Aim 1 of this project will quantify longitudinal changes over 3 years in a cohort of 50 patients. Development of a DM2-specific patient-reported outcome measure will be completed and validated. Aim 2 will perform comprehensive transcriptome comparison in DM1 and DM2, and test the hypothesis that muscle weakness in DM2 shows the same association with RNA spliceopathy as in DM1. Alternatively, we will determine whether RAN translation occurs in DM2 skeletal muscle, as was recently observed in the CNS. Aim 3 will complete the clinical steps of qualifying RNA splicing biomarkers as drug development tools for DM1, using targeted high throughput sequencing to assess a panel of 22 splice events. In this Aim we will assess disease specificity of splicing changes, make comparisons between muscles that are selectively affected versus those that are relatively spared, and determine whether the same set of splicing biomarkers developed for DM1 can also be applied to DM2. Overall, this project will supply critical information that is needed to move forward with therapeutic development for DM1 and DM2.